1. Field of the Invention
The present invention relates to a printer controller for creating and delivering printing data (raster data) which is received by a color image forming device for forming an image.
2. Related Art
Recently, full color printer devices using an electrophotographic process or an ink jet system have been prosperously used as office printers, that is, output terminal units for personal computers, workstations or the like.
Referring to FIG. 5 which is a configuration view illustrating a color printer in general, there are shown a personal computer 30, a printer 31, a printer controller 100 which is one of components of the printer 31, and a color image forming device 200 which is also one of the components of the printer 31.
As shown in FIG. 5, printing in the printer 31 using a electrophotographic system is carried out in such a way that printing data created by the personal computer 30 is received through an interface by the printer 31 for forming an image. It is noted that a CAD, a workstation or a digital camera may be used, instead of the personal computer 30.
The configuration of the printer 31 will be detailed. As shown in FIG. 5, the printer includes therein the printer controller 100 and a printer engine 200.
Referring to FIG. 6 which is a block diagram illustrating the printer controller 100, there are shown a receiving part 101 for receiving printing data, an interpreter part 102 for interpreting languages for the printing data, a rasterizing part 103 for converting the interpreted printing data into raster data, a compression part 104 for compressing the raster data and for delivering a compressed raster data, a memory 105 for storing the compressed raster data, an expansion part 106 for restoring the compressed raster data, and a data transmission part 107 for transmitting the restored data to the color image forming device (printer engine) 200.
As shown in FIG. 6, in the printer controller 100, the receiving part 101 receives printing data such as data including print language, bit map data or mixture thereof, concerning a document to be printed. The printing data in the form of the print language is interpreted by the interpreter part 102, and is converted by the rasterizing part 103 into raster data with which the printer engine 200 forms an image. As to the volume of raster data for four-color printing, estimating that the degree of resolution is 600 dpi while the size of a printing sheet is A3, and a gradation is binary, the data size becomes 35 Mbytes. Further, should the degree of resolution be increased, or should data for a plurality of pages be once rasterized, the volume of raster data would be extensive, and accordingly, a mass memory for temporarily storing the rater data would be required. Thus, there has been in general used such a technique that the volume of the raster data is reduced by the compression part 104 so as to decrease the volume of the data. The data which has been compressed is once stored in the memory 105, and the compressed data is then restored by the expansion part 106 before the data is transmitted to the printer engine 200 through the data transmission part 107.
Meanwhile, the printer engine 200 carries out image formation after a latent image is formed in accordance with the raster data delivered from the printer controller 100.
Referring to FIG. 7 which is a schematic view illustrating a printer engine 200, there are shown image forming stations 1a to 1d, photosensitive media 2a to 2d, chargers 3a to 3d, developing units 4a to 4d, cleaning units 5a to 5d, exposure units 6a to 6d, transfer units 8a to 3d, exposure beams 9a to 9d, support rollers 10, 11, an intermediate transfer belt 12 serving as a image bearing medium, raster data receiving parts 13a to 13d, a pattern detecting part 14, a paper feed cassette 16, sheet materials 17, a paper feed roller 18, a sheet material transfer roller 19, a fixing unit 20, a skew/magnification detecting part 32, and an image forming light drive control part 35.
As shown in FIG. 7, the printer engine 200 includes a plurality of image forming stations 1a to 1d for carrying our processes of forming images so as to form color images such as a cyan image, a magenta image, a yellow image and a black image which is preferable, on the intermediate transfer belt 12, and then, the images on the intermediate transfer belt 12 are transferred onto a sheet material 17, being superposed with one other, at an image transfer position so as to form a full color image. The above-mentioned printer engine which is a tandem type has been proposed. Such a tandem type printer engine is advantageous in view of speed-up of the image formation since the plurality of image forming parts are provided for the color images, respectively.
However, there has been raised such a problem that how the color images which are formed respectively in the different image forming parts can be satisfactorily aligned with one another because deviation among four color image forming positions causes positionnal deviation or color tone variation, finally.
Next, the positional deviation among transferred images will be explained with reference to FIGS. 8a to 10. FIG. 8a is a view for explaining a positional deviation in the travelling direction of the transfer medium, that is, in the direction of the arrow A in the figure, (this positional deviation will be referred to “vertical scanning positional deviation”), FIG. 8 is a view for explaining a positional deviation in a scanning direction, that is, the direction orthogonal to the direction of the arrow A, (this positional deviation will be referred to “horizontal scanning positional deviation), FIG. 8c is a view for explaining a positional deviation in a skew direction (this positional deviation will be referred to “skew error”), FIG. 8d is a view for explaining a magnification error, and FIG. 8e is a view for explaining a curving error. FIG. 9 is a block diagram illustrating parts relating to correction for skew and magnification in the color image forming device, and FIGS. 10 to 10c are views for explaining correction for skew and magnification errors in a conventional printer engine.
Referring to FIG. 9, there are shown a printer controller 100, a color image forming device 101, a skew/magnification detecting part 32 for transmitting a skew error and a magnification error, a skew/magnification correction coefficient computing part 33 for computing a skew correction coefficient and a magnification correction coefficient from detected errors, a skew/magnification correcting part for correcting the skew errors and the magnification error in accordance with the skew correction coefficient and the magnification correction coefficient, an image forming beam drive control part 35 for driving and controlling an image forming beam, and a line memory for storing therein data.
As to the kinds of positional deviations in a transferred image, as shown in FIGS. 8a to 8e, there are presented the vertical scanning positional deviation (Refer to FIG. 8a), the horizontal scanning positional deviation (Refer to FIG. 8b), the skew error (FIG. 8c), the magnification error (Refer to FIG. 8d), and the curving deviation (Refer to FIG. 8e). However, the above-mentioned five kinds of deviations and errors are superposed with one another, being actually exhibited.
Further, as to main causes of the above-mentioned positional deviations, the vertical scanning positional deviation (Refer to FIG. 8a) is caused by installations errors of the image forming stations and the scanning optical systems, and as well, by installation errors of lenses and mirrors (which are not shown) in the scanning optical systems, and the horizontal scanning positional deviation (Refer to FIG. 8b) rly caused thereby. Accordingly, these deviations can be simply eliminated, for example, by electrically adjusting the timing of the exposures 6a to 6d of raster data.
The skew error shown in FIG. 8c is caused by angular errors of the rotary shaft of a photosensitive drum and angular installation error of the scanning optical systems, in the image forming station. The magnification errors (Refer to FIG. 8d) is caused by deviations in the scanning length due to the optical lengths from the scanning optical systems to the photosensitive drums in the image forming stations, and the curving error (refer to FIG. 8e) is caused by errors of the assembly of the lenses in the scanning optical systems.
Accordingly, there has been proposed the following correcting manner: the above-mentioned five kinds of deviations are detected from reference patterns (register patterns) which have been drawn previously, by means of the skew/magnification detecting part 32 (detection of a positional deviation), and then, degrees of alignment (skew values and magnifications) for images in accordance with the deviations are calculated from the result of the detection by the skew/magnification correction coefficient computing part 33 so that correction is made by the skew/magnification correcting part 34.
FIGS. 10a to 10c show the manner of this correction. Referring to FIG. 10a, a rectangle to be drawn, indicated by the chain line is actually depicted as being indicated by a solid line due to a skew error and a magnification error with no correction. Accordingly, pixels are shifted in the vertical scanning direction on the basis of a skew correction angle α as shown in FIG. 10a, and thereafter, the pixels are enlarged (reduced) in the main scanning direction on the basis of a magnification correction rate β as shown in FIG. 10c. It is noted that FIGS. 10a to 10c are drawn, being exaggerated more or less in order to clarify the skew correction. However, actually, the correction is made with a minimum unit of the resolution of vertical scanning.
Should the correction for deviations and errors shown in FIGS. 10a to 10c be made, as mentioned above, in the printer engine 200, the line memory 36 would be required for storing data having a volume corresponding to a number of lines for the correction in the vertical scanning direction of printing, as shown in FIG. 9.
As mentioned above, the conventional printer controller has raised such a problem that a line memory 36 is additionally required for produced raster on the printer engine side in order to correct a skew and a magnification of the printer engine 200.
In this printer controller, it is required to eliminate the necessity of a line memory, by correcting skew and magnification errors within the printer controller.